Flame detection apparatus

ABSTRACT

A gas flame produces a voltage between the burner and an electrode in the flame. This voltage is utilized in a circuit which produces a pulse signal to control a relay that indicates flame presence. Failure of any critical component in the circuit prevents the relay from operating.

United States Patent Eberle et a1.

[ 1 June 20, 1972 FLAME DETECTION APPARATUS [56] References Clted Inventors: Arthur C. Eberle, Lee H. Darby, both of UNITED STATES PATENTS 4322' V 3,302,685 2/1967 Ono et al. ..34o/22s.1 Assignee: Columbia Gas System Service Corporation, 33011307 1967 Nlshlgakl e1 340/2281 WM 2,519,789 8/1950 Perkins ..328/6 Filed: June 20 1972 Primary Examiner-L. T. HiX

Assistant Examiner-Harry E. Moose, Jr. Appl. No.: 143,326 V Attorney-Curtis, Morris & Safford [57] ABSTRACT U.S.Cl. .317 133. 317 132, 32

' l 8/6 A gas flame produces a voltage between the burner and an electrode in the flame. This voltage 15 utilized in va circuit Ill. Cl. ..H0lh 47/26 which produces a pulse signal to control a relay that indicates ofSearch ..3l7/133.5, flame presence Failure of any critical component in [he Cir.

340/228. 1; 328/6; 310/4 R; 322/2 R cuit prevents the relay from operating.

5 Claims, 1 Drawing Figure LE 1 L 52 \O 22 L857 2. i.

PATENTEDJUNZO 1972 3,671,815

INVENTOR.

Arthur C. Eberle BY LEE H. DARby fimg/ilm W ATTORNEYS FLAME DETECTION APPARATUS This invention relates to a flame-actuated circuit, and more particularly to a flame relay circuit which utilizes the principle of flame generation of voltage in controlling a relay.

There is a continuing need for monitoring devices on fuel burning appliances which are capable of detecting the presence of a flame. Such devices are especially desirable when the fuel used is a gas. Once the pilot flamedisappears in a gas furnace or appliance, it is essential that the gas supply be turned ofi promptly to insure maximum safety and economy. Therefore, it is highly desirable that any monitoring device have a rapid response time.

Heat sensing devices utilizing bimetallic elements, rod and tube sensors, and mercury filled elements as monitoring devices have limited application because of their long response time. Some flame-proving devices, long in use, utilize the principles of flame conduction or flame rectification. The flame conduction principle depends on electrical conduction through the ionized portion of a flame. Because some ionized gas lingers after flame failure, the reaction time of monitoring devices using this principle is relatively long. Furthermore,if the sensing electrodes in a flame conduction device become contaminated or shorted, a false indication of flame presence can occur. Flame rectification devices, although more dependable, are expensive and their effectiveness is dependent on several variables, such as, critical electrode positioning, burner configuration, flame geometry and burner material.

Another type of monitoring device utilizes the thermo-couple effect of dissimilar metals to generate a voltage which is indicative of flame presence. These devices, however, have a relatively long response time and suffer from aging due to mechanical degradation from heat.

The present invention uses the principle of flame generation of voltage to overcome the limitations of the monitoring devices referred to above. According to this principle, a flame adjusted so that its base is in contact with a metal burner will generate a voltage between the burner and a metallic electrode inserted into the flame. This potential can be as much as two volts when measured across a resistance of several megohms. Other tests show that the positioning of the metallic electrode is less critical than the positioning of the electrode or electrodes associated with flame rectification devices. Flame generation is also possible with ceramic plate burners if a wire screen, grounded to the burner piping, is placed between the sensing electrode and the burner.

In the illustrative embodiment of the present invention, there is a rapid voltage response to burner flame-out, and this voltage response can be used to cause valve closure in less than one second after the flame-out. The electrical potential is created by the flame not by ionized gas. The voltage stops instantly when the flame. goes out, and as a direct result, there is a constant and vigilant determination as to the presence of the flame.

The circuit of the illustrative embodiment includes a capacitor connected to the burner and to the electrode to be charged by the flame-generated voltage. The source and drain electrodes of a field effect transistor, frequently called a PET, are connected to the capacitor to discharge it when the PET is caused to conduct. The FET is biased to be non-conductive but its gate is connected to an alternating current source that triggers the FET to make it conductive once each cycle. A resistor in series with the capacitor has a voltage pulse generated across it by the capacitor discharge current, and this voltage pulse is amplified and used to trigger a silicon controlled rectifier, called an SCR, into conductivity that lasts for part of a cycle of the alternating current.

The SCR derives its operating current from the charge stored in a second capacitor which is connected to the alternating current source through a half wave rectifier. The SCR and a relay are connected in series across this second capacitor to drain off the charge through the relay coil when the SCR conducts. The second capacitor should be completely discharged before the end of a half cycle of the alternating nected to an alternating current source 29 which current and the SCR should be non-conductive by the time the half wave rectifier is able to supply a new charge to the second capacitor. The circuit includes a fuse to guard against the possibility that the SCR may remain conductive too long and may draw excessive current directly from the alternating current source. The same fuse guards against short circuiting of other components in the circuit.

It is an object of the present invention to provide a improved circuit actuated by flame generated voltage. In particular it is an object of the' present invention to provide a flame-actuated relay for rapid indication of the existence or loss of a flame.

Further objects will be apparent from the following specification together with the drawing in which the only F IGURE is a circuit'diagram of a flame-actuated-relay constructed according to the invention. 1

The flame'relay shown in'the circuit diagram has two input terminals 11 and 12m be connected, respectively, to a probe 13 that extends into a flame l6 and to a burner 14. It is assumed that the burner is conductive but if another type of burner, such as a ceramic burner, is used, the terminal 12 can be connected'to'a screen that is grounded or is connected to the piping that supplies the burner. In any case, there must be another conductive member in addition to the probe 13 and located in such a way that the flame 16 of the burner extends between the probe 13 and the other conductive member so that voltage generated by the flame will produce a difference of potential between the probe 13 and the othermember, such as the burner 14. This voltage, in turn, is supplied across the input terminals 1 1 and 12 of the circuit.

The terminal 12 is grounded and the terminal 11 is connected by way of the resistor 17 to the source terminal of a field effect transistor 18. The drain terminal of this field effect transistor is connected to ground. The source and drain terminals of the field efiect transistor 18 are also connected across a series circuit comprising a capacitor 19 and a resistor 21.

The gate of the field effect transistor 18 is biased by a direct current source that comprises a rectifier 22 and a smoothing circuit that includes a resistor 23 and a capacitor 24. The rectifier 22 is connected by way of a fuse 26 to one of the altemating current input terminals 27 of the circuit. The other alternating current input terminal is indicated by reference numeral 28 and these two terminals are adapted to be conmay be any suitable voltage such as 24 volts alternating current at a frequency of 60 Hz. The direct current output terminal of the rectifier circuit is indicated by reference numeral 31 and is connected to one end of a series circuit comprising a resistor 32 and a second resistor 33. The latter is grounded and the gate of the field eflect transistor 18 is connected to the junction between the resistors 32 and 33.

The gate of the field effect transistor is also supplied with a modified alternating voltage by way of another series circuit comprising a resistor 34'and a bi-lateral switch 36 that is conductive only on voltage peaks. This switch is connected directly to the fuse 26.

The emitter of a transistor 37 is connected to the junction of the capacitor 19 and the resistor 21. The base of thistransistor is held at a pre-determined bias level by a resistor 38 connected to the junction between another resistor 39 and a capacitor 41. The collector of thetransistor 37 has a load re- 7 sistor 42 and is connected by way of a capacitor 43 to the base of a second transistor 44. The collector of the latter transistor is connected by way of a capacitor 46 back to the base of the transistor 37. The emitter of the transistor 44 is connected to ground by a resistor 47 and the base of this transistor is connected to ground by a resistor 48: The capacitor 49 is con nected across the resistor 48 to stabilize the operation of the circuit. The collector of the transistor 44 is connected to the output terminal of the direct current source through a collector load resistor 51.

Another rectifier 52 is also connected to the fuse 26 and to a capacitor 53 the other side of which is grounded. In parallel with the capacitor 53 is a series circuit comprising the coil of a relay 54 and the main anode-cathode circuit of an SCR 56. The gate of the SCR 56 is connected directly to the emitter of the transistor 44. The relay 54 is indicated as having normally open contacts 57 and there is a diode 58 connected across the coil of the relay to damp out reverse voltage pulses.

The operation of the circuit is controlled by the presence or absence of the flame 16. As long as the flame is present, it produces a voltage difference between the terminals 11 and 12 that charges the capacitor 19 by way of the resistors 17 and 21. During the peak part of one half of each alternating current cycle of the source 29, the bi-lateral switch 36 becomes conductive. This could occur during either the positive or negative peak of voltage from the source 29, but the FET will be conductive only on the peak part of the negative half cycle of the alternating current from the source 29. When the FET becomes conductive, it discharges the capacitor 19 through the resistor 21, thereby generating a small positive pulse across the resistor 21.

The positive pulse at the junction between the capacitor 19 and the resistor 21 tends to turn 06 the normally-conductive transistor 37, thereby causing its collector to become more positive. This applies a positive-going voltage to the base of the transistor 44 and causes that transistor to begin conducting. The conductivity of the transistor 44, in turn, causes a negative swing of its collector, and this negative swing of its collector is transmitted by the capacitor 46 back to the gate of the transistor 37 to turn the latter off completely.

The regenerative action between the transistors 37 and 44 produces a positive going voltage at the emitter of the transistor 44 and this voltage is applied to the gate of the SCR 56 causing the latter to become conductive. When it does, it draws current from the charged capacitor 53 through the coil of the relay 54 and produces a magnetic field that closes the contacts 57. These contacts remain closed as long as the capacitor 53 is able to supply current through the relay 54 and the SCR 56. However, there are limits to the amount of charge stored in the capacitor 53, and the value of capacitance is selected so that the current produced by the charge stored in it is dissipated before the end of the half cycle of the alternating current from the source 29. As a result, the SCR 56 becomes non-conductive for lack of available current before the positive half of the alternating current begins. Thus the SCR 56 draws current only from the capacitor 53 and not by way of the diode 52, except to the extent that the current flowing through the diode 52 during positive half cycles is the current that supplies the charge to the capacitor 53.

In the absence of a flame 16, there is no voltage to charge the capacitor 19. Therefore, when the FET 18 becomes conductive, no voltage will be generated across the resistor 21 and the regenerative circuit formed by the transistors 37 and 44 will not supply the necessary voltage to turn on the SCR 56.

This method of detection of a flame works because the sourcedrain current of the FET has no junction barriers to pass through that would cause voltage off-sets of the type that exist across semi-conductor junctions therefore the operation of this circuit would not be possible with conventional bi-polar transistors in place of the F ET 18.

The no-flame condition will be indicated by the fact that the relay contacts remain open, and this can be indicated by a light or can control a valve that will close off the fuel supply to the burner 14 if the contacts 57 do not close once each cycle of the alternating current.

What is claimed is:

1. A flame detection circuit comprising first and second terminals to be connected conductively to a flame; a field effect transistor having gate, source and drain electrodes, said first and second terminals being connected in series with said source and drain electrodes; a first series circuit comprising a capacitor and a resistor connected in series with said first and second terminals wherebyl said capacitor can be charged by voltage generated by said ame; means to apply current pulses to said gate of said field effect transistor to render the latter conductive at repetitive intervals to discharge said capacitor; regenerative means connected to said series circuit to be actuated by discharge current from said capacitor; and relay means connected to said regenerative means to be actuated thereby.

2. The flame detection circuit of claim 1 comprising, in addition, a second capacitor; a rectifier to connect said second capacitor to a source of alternating voltage to be charged during alternate half cycles of said alternating voltage; and a silicon controlled rectifier having an anode-cathode circuit connected in series with said relay means and said second capaci' tor, said silicon controlled rectifier also having a gate connected to said regenerative means to be actuated thereby to cause said silicon controlled rectifier to discharge said second capacitor through said relay means when there is a flame.

3. The flame detection circuit of claim 2 in which said regenerative means actuates said silicon controlled rectifier to discharge said second capacitor only during alternate half cycles of said alternating voltage when said rectifier is not conductive.

4. The flame detection circuit of claim 1 in which said means to apply current pulses to said gate of said field effect transistor comprises a bilateral switch connected to a source of alternating current to supply said current pulses to said gate of said field effect transistor during peaks of said alternating current to supply said current pulses to said gate of said field effect transistor during peaks of said alternating current.

5. The flame detection circuit of claim 4 comprising, in addition, a second rectifier connected to said source of alternating current to generate a direct voltage and connected to the gate of said field effect transistor to provide direct bias therefor. 

1. A flame detection circuit comprising first and second terminals to be connected conductively to a flame; a field effect transistor having gate, source and drain electrodes, said first and second terminals being connected in series with said source and drain electrodes; a first series circuit comprising a capacitor and a resistor connected in series with said first and second terminals whereby said capacitor can be charged by voltage generated by said flame; means to apply current pulses to said gate of said field effect transistor to render the latter conductive at repetitive intervals to discharge said capacitor; regenerative means connected to said series circuit to be actuated by discharge current from said capacitor; and relay means connected to said regenerative means to be actuated thereby.
 2. The flame detection circuit of claim 1 comprising, in addition, a second capacitor; a rectifier to connect said second capacitor to a source of alternating voltage to be charged during alternate half cycles of said alternating voltage; and a silicon controlled rectifier having an anode-cathode circuit connected in series with said relay means and said second capacitor, said silicon controlled rectifier also having a gate connected to said regenerative means to be actuated thereby to cause said silicon controlled rectifier to discharge said second capacitor through said relay means when there is a flame.
 3. The flame detection circuit of claim 2 in which said regenerative means actuates said silicon controlled rectifier to discharge said second capacitor only during alternate half cycles of said alternating voltage when said rectifier is not conductive.
 4. The flame detection circuit of claim 1 in which said means to apply current pulses to said gate of said field effect transistor comprises a bilateral switch connected to a source of alternating current to supply said current pulses to said gate of said field effect transistor during peaks of said alternating current to supply said current pulses to said gate of said field effect transistor during peaks of said alternating current.
 5. The flame detection circuit of claim 4 comprising, in addition, a second rectifier connected to said source of alternating current to generate a direct voltage and connected to the gate of said field effect transistor to provide direct bias therefor. 